1. Field of the Invention
The present invention relates to a chip type electronic part, and more particularly to an outer electrode structure of a chip type electronic part appropriate mainly for reflow soldering.
2. Description of the Prior Art
A chip type electronic part for example, a monolithic ceramic capacitor, has a construction where a capacitor chip 2 is formed by laminating and baking a plurality of ceramic sheets provided with an internal electrode 1, and both ends of the capacitor chip 2 are each provided with an outer electrode 3 connected with the internal electrode 1 as shown in FIG. 2. In mounting the capacitor onto a substrate, the outer electrode 3 is soldered to a circuit.
In soldering chip, mainly the reflow soldering is adopted. The reflow soldering is a method of preliminarily supplying solder such as cream solder onto a substrate, mounting a chip type electronic part on the substrate, and then melting the solder by applying heat to all or a part of the substrate to perform soldering.
The outer electrode 3 has been conventionally made of silver (Ag) or an alloy thereof (Ag-Pd), and composed of an electrode body 4 mounted at the end portions of the capacitor chip 2 as connected with the internal electrode 1. A protection layer 5 is formed of a plating coat made of nickel (Ni) covering an outer surface of the electrode body 4 to prevent the electrode body 4 from being eaten by the solder, and an outermost layer 6 is provided so as to cover an outer surface of the protection layer 5. The outermost layer 6 is made of tin (Sn) or an alloy thereof (Sn-Pb) having good solderability because the protection layer 5 has poor solderability. The outer electrode 3 is fixed by soldering onto a circuit 8 of a substrate 7 with solder 9.
Since the outermost layer 6 of the above-mentioned outer electrode 3 is made of tin or an alloy thereof, there can be achieved good solder wettability and sufficient solder adhesion by melting the solder at a constant temperature to consequently achieve firm solidification.
The solder 9 is generally formed of a eutectic solder composed of 63 percent of tin and 37 percent of lead. Note that the melting point of the above-mentioned solder 9 is 183.degree. C., the melting point of tin (100% Sn) serving as the outermost layer 6 is 232.degree. C., and the alloy of tin and lead (90% by weight of Sn and 10% by weight of Pb) has a solidus temperature of 183.degree. C.
When the soldering is performed at the melting point of the outermost layer 6 in the aforementioned soldering process, the outermost layer 6 requires a long time to melt and is hardly melted, which causes defective soldering due to the unmelted outermost layer 6.
For the above reason, the soldering has been conventionally performed at a high temperature of about 240.degree. C. to 260.degree. C., which is higher than the melting point of the outermost layer 6.
However, soldering at such a high temperature has a problem is that a thermal influence is exerted on the other electronic parts mounted on the substrate or the substrate itself, which causes high-density mounting of parts on a substrate to be difficult.
When the soldering is performed at a low temperature of not higher than 230.degree. C. to prevent such a harmful influence of the high-temperature soldering, defective soldering due to the unmelted outermost layer 6 takes place, and, furthermore, a difference in solder wetting speed tends to take place between the outer electrodes at both the end portions in the soldering process by ambient atmosphere reflow. Consequently, since the soldering is not performed simultaneously at both the end portions, the ceramic capacitor stands erect on the substrate, i.e., the so-called "tombstone phenomenon" occurs disadvantageously.